The present day demand on the offshore oil and gas industry for greater safety and efficiency is the strongest in the history of the industry. This demand makes it necessary for equipment to be sufficiently versatile to be economically and operationally efficient without compromising safety to personnel, equipment, or the well. Drilling of the well is the primary function in obtaining oil and gas. Subsequent well workover or servicing is necessary to realize the maximum product output potential of the well. The workover or servicing is normally done continually at regular intervals, such as annually or biannually, over many years. Some wells have been known to produce over a forty-year period.
Offshore industry requirements have led to the development and construction of various special purpose marine structures capable of continuous operation in the offshore environment for extended periods of time. For example, one such structure that is commonly employed is a fixed, self-contained drilling and production platform erected on piles driven into the sea floor. A drilling workover or service rig, auxiliary equipment, and crew quarters are mounted on the platform. A commonly used variation of this type of structure is a smaller platform similarly erected on piles and having a drilling, workover, or servicing rig located thereon. The auxiliary equipment and crew quarters are located on a separate tender barge tied alongside.
Mobile tender vessels are required to erect a rig in the offshore environment, to dismantle the rig in the case of discontinued drilling, and to carry out workover or servicing. Conventional tender vessels take the form of barges on which are mounted heavy duty derricks or cranes. The cranes are used to lift, transfer, and set into place the parts forming the rigs. A tender barge typically has space on board to transport the rig parts, and the barge-mounted crane is used to lift and set the rig parts onto the platform.
Conventional tender barges employed for erecting, dismantling, and servicing oil and gas drilling and production platforms are single hull surface floating vessels that are either towed or self propelled to and anchored at the platform site. This type of tender vessel has the major disadvantage of being highly restricted in terms of the sea conditions in which it can be used for rig erecting and dismantling and servicing operations. In sea states with even moderately high wave heights, the tender vessels have a high level of motion in heave, pitch, and roll and a tendency toward abrupt correcting motions when the vessel is subjected to roll or pitch excitations. For example, surface floating tender barges currently employed for offshore operations generally are restricted to operation in sea states having wave heights of no more than five to six feet. The wave action against the vessel caused by sea states having wave heights in excess of these limits normally causes vessel motion at a level sufficiently high to create a risk of structural or wire failures in the vessel's crane. This risk precludes crane operations. Thus, operations utilizing conventional barges are normally halted when high sea state conditions are encountered and are resumed only when the sea state subsides to within the limits.
The above-discussed shortcomings of conventional tender barges have been recognized in the industry for a number of years. There have been proposals for overcoming the shortcomings by providing a semisubmersible vessel having a working platform supported on columns extending upwardly from a pair of hulls that may be ballasted to submerge the hulls. U.S. Pat. No. 3,835,800, granted Sep. 17, 1974, to S. H. Lloyd III et al., discloses a semisubmersible derrick barge in which a heavy duty derrick or crane is mounted at one end of the barge on the longitudinal centerline of the barge. The disclosed barge is also the subject of U.S. Pat. No. 4,165,702, granted Aug. 28, 1979, to the same inventors. Other proposals for semisubmersible vessels are found in U.S. Pat. No. 3,894,503, granted Jul. 15, 1975, to A. C. McClure; U.S. Pat. No. 4,207,828, granted Jun. 17, 1980, to A. Horowitz et al; U.S. Pat. No. 4,231,313, granted Nov. 4, 1980, to P. S. Heerema et al.; U.S. Pat. No. 4,232,625, granted Nov. 11, 1980, to Y. Goren et al.; and U.S. Pat. No. 4,281,615 and U.S. Pat. No. 4,471,708, granted Aug. 4, 1981, and Sep. 18, 1984, respectively, to J. H. Wilson et al. Each of these patents discloses a tender barge, except the Goren et al. patent, which discloses a semisubmerged drilling vessel. Also of interest is U.S. Pat. No. 3,685,305, granted Aug. 22, 1972, to S. H. Lloyd III, which discloses a pipeline laying barge.